1. Field of the Invention
The present invention relates to a system for diagnosing the fuel supply system of an internal combustion engine.
2. Related Art
In a diagnosis system for a fuel supply system according to JP-A-4-171237, a learned correction coefficient for correcting deviation of an actual air-fuel ratio from a target air-fuel ratio is used for diagnosing the fuel supply system. The learned correction amounts are averaged for each operating zone in which basic fuel injection amounts are generally unchanged so much and the average value of the learned correction amounts is compared with the average value of the learned correction amounts of an operating zone having a different basic fuel injection amount, thereby diagnosing the fuel supply system.
In another diagnosing system for a fuel supply system according to U.S. Pat. No. 5,094,214, a learned correction amount and a feedback correction amount of an air-fuel ratio are used as diagnostic data and the fuel supply system is diagnosed by discriminating whether or not a state in which both the learned correction amount and the feedback correction amount reach the upper and lower limit values (guard values) of an air-fuel ratio control range continues for a predetermined time.
According to those diagnosing systems for the fuel supply system, the learned correction amount is used as diagnostic data. The learning or updating speed of the learned correction amount is, however, generally set to be slow in order to prevent erroneous learning. Consequently, the period until the learned correction amount is updated is relatively long. Even if a malfunction or abnormality occurs in the fuel supply system during this period, the malfunction of the fuel supply system cannot be detected until the learned correction amount is updated. When the air-fuel ratio is equal to or larger than the feedback correction amount, there is a case such that the updating of the learned correction amount is inhibited. In this case, malfunction of the fuel supply system cannot be detected until the learned correction amount becomes equal to or lower than the feedback correction amount. In short, those diagnosing systems have a drawback such that the diagnosing operation is influenced by the updating time of the learned correction amount and malfunction of the fuel supply system cannot be promptly detected.